The optical interface between a light source and a detector is important. In most practical realizations the aim should be to collect the maximum light possible. Lens systems seldom do this efficiently, especially where the light source is diffuse. Underlying any attempt at concentrating or guiding light is subject to a fundamental limitation referred to as étendue (phase space cannot be squeezed). Light collection from small volume scintillators of high refractive index may approach 50 %, while collection from large-area scintillators is typically less than a few per cent. Incorporation of wavelength-shifting light guides and fibres leads to enhanced performance. Efficiency measurements by the author in terms of photoelectrons per keV are presented for selected configurations. Optical recycling derived from total internal reflection provides enhancement in effective quantum efficiency by a factor of up to 10. Concepts such as escape cones, adiabatic light guides, and trapped light are covered in detail.
Noncontact Optical Fiber Sensor for Measuring the Refractive Index of Liquids
